1. Field of the Invention
The present invention relates to a fault detector for detecting faults in a DC capacitor circuit incorporated into an inverter circuit which converts direct current into alternating current by means of semiconductor devices.
2. Description of the Prior Art
Generally, internal short-circuit and capacity reduction due to time aging are the most likely faults in a DC capacitor. The internal short-circuit of the capacitor causes a variation of the DC voltage of the inverter circuit, and hence the inverter circuit is provided with fuses to protect the related equipments and circuits.
FIG. 1 is a circuit diagram of a fault detecting circuit for detecting faults in the DC capacitor of an inverter, introduced in "Electric Power and Railroad", Vol. 28, No. 6 (278), pp. 29-31. As illustratred in FIG. 1, this known inverter circuit 8 comprises DC capacitors 1 and 2 connected in series, voltage dividing resistances 3 and 4 for equalizing the respective voltages of the DC capacitors 1 and 2, fuses 5 that melt and break the circuit when the DC capacitors 1 and/or 2 are short-circuited, fuse fusion detectors 6 and DC buses 7 of the inverter circuit 8. This inverter circuit 8 comprises a plurality of component circuits each comprising, as the principal elements, the DC capacitors 1 and 2, and the fuse 5, and connected in parallel to the DC buses 7.
The manner of operation of this known inverter circuit 8 will be described hereinafter. During a normal state when the DC capacitors 1 and 2 are not short-circuited, the fuse fusion detectors 6 do not operate. Suppose that one of the DC capacitors 1 and 2 short-circuits. Then, the voltage of the busses 7, namely, a voltage twice the normal voltage, is applied to the other DC capacitor. In some cases, the abnormal voltage exceeds the rated voltage of the DC capacitor, which, in some cases, causes the latter DC capacitor to short-circuit, and thereby an overcurrent flows through the DC capacitors 1 and 2, and the fuse 5, so that the fuse 5 melts and breaks the circuit. Then, the fuse fusion detector 6 of the inverter circuit 8 detects the fusion of the fuse 5.
Since the conventional fault detector for detecting faults in a DC capacitor circuit is thus constituted, a time of several tens of milliseconds to several hundreds of milliseconds elapses from the moment of occurrence of the short-circuit of the busses 7 of the inverter circuit 8 before the fuse fusion detector 6 functions delaying the detection of the short-circuit of the DC buses 7. Thus, the conventional fault detector is incapable of instantaneous detection of such a fault.
Furthermore, the above-mentioned conventional fault detector is designed to detect only the internal short-circuit of the DC capacitor, and hence the fault detector is incapable of dealing with the other fault, namely, the variation of the capacitance of the DC capacitor circuit attributable to time aging.